fix(cmake): Use standard install dirs on Linux

The previous method of installation didn't respect the standard dirs,
leading to build tools not recognizing DPP correctly. This commit fixes
this by moving everything to the standard dirs. As a side effect, files
are no longer installed twice, due to CMake already placing some files
in the right spots.

Issue: brainboxdotcc#590
Issue: brainboxdotcc#335

cmake/CPackSetup.cmake

@@ -1,9 +1,10 @@
 include(GNUInstallDirs)
 set(DPP_EXPORT_NAME dpp)
+set(DPP_CMAKE_DIR ${CMAKE_INSTALL_LIBDIR}/cmake/${DPP_EXPORT_NAME})
 set(DPP_VERSIONED ${DPP_EXPORT_NAME}-${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR})
 set(DPP_VERSION_FILE ${PROJECT_BINARY_DIR}/${DPP_EXPORT_NAME}-config-version.cmake)
-set(DPP_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR}/${DPP_VERSIONED})
-set(DPP_INSTALL_LIBRARY_DIR ${CMAKE_INSTALL_LIBDIR}/${DPP_VERSIONED})
+set(DPP_INSTALL_INCLUDE_DIR ${CMAKE_INSTALL_INCLUDEDIR})
+set(DPP_INSTALL_LIBRARY_DIR ${CMAKE_INSTALL_LIBDIR})
 
 ## Pack the binary output
 install(TARGETS dpp
@@ -19,15 +20,11 @@ write_basic_package_version_file(${DPP_VERSION_FILE}
 		VERSION ${PROJECT_VERSION}
 		COMPATIBILITY SameMajorVersion)
 
-## Package the include headers (the trailing slash is important, otherwise
-## the include folder will be copied, instead of it's contents)
-install(DIRECTORY "${CMAKE_SOURCE_DIR}/include/" DESTINATION "${DPP_INSTALL_INCLUDE_DIR}")
-
 ## Include the file which allows `find_package(libdpp)` to function.
-install(FILES "${CMAKE_SOURCE_DIR}/cmake/libdpp-config.cmake" "${DPP_VERSION_FILE}" DESTINATION "${DPP_INSTALL_LIBRARY_DIR}")
+install(FILES "${CMAKE_SOURCE_DIR}/cmake/libdpp-config.cmake" "${DPP_VERSION_FILE}" DESTINATION "${DPP_CMAKE_DIR}")
 
 ## Export the targets to allow other projects to easily include this project
-install(EXPORT "${DPP_EXPORT_NAME}" DESTINATION "${DPP_INSTALL_LIBRARY_DIR}" NAMESPACE dpp::)
+install(EXPORT "${DPP_EXPORT_NAME}" DESTINATION "${DPP_CMAKE_DIR}" NAMESPACE dpp::)
 
 # Prepare information for packaging into .zip, .deb, .rpm
 ## Project installation metadata